This invention relates to an optical disk access method for searching for an intended target track among numerous tracks formed on an optical disk, and particularly to an optical disk access method and an optical disk storage using this method in which a light spot is positioned to an intended target track by use of a coarse actuator and a fine actuator.
There has been developed the optical disk storage in which high density information is recorded and reproduced on a rotary recording medium which can also be erased when necessary. The rotary recording medium, i.e., optical disk, is provided with numerous recording tracks at a constant track pitch in the form of concentric circles or a spiral, and each track is segmented into several sectors for partitioning stored data. In recording, retrieving or erasing information at an arbitrary location on the disk, the disk storage is subjected to the access operation (seek operation) for finding the intended track among numerous tracks on the disk surface and then finding the intended sector on the track. The operation involves a coarse seek control which quickly moves the light spot to the vicinity of the target track, a tracking control which maintains the light spot on the center of the track, and a fine seek control (jump control) which corrects the deviation of the light spot from the target track. The above optical disk access operation is described in Japanese Patent Unexamined Publication Nos. 58-91536 and 58-169370 (equivalent to U.S. patent application Ser. No. 736,125, now U.S. Pat. No. 4,607,358, which is a continuation application of U.S. patent application Ser. No. 443,399, abandoned; EPC Application No. 821109071).
The above-mentioned optical disk storage implements the light spot positioning control by using a coarse actuator such as a linear motor for moving the optical head and a fine actuator, such as a galvanomirror or a voice coil for driving an objective lens, mounted on the optical head. The coarse positioning for the optical recording/reproduction head to a track on the optical disk uses as a position sensor an external scale such as an optical linear scale fixed on the base. The external scale has a scale pitch equal to a multiple of the track pitch. After the coarse seek operation has settled, a temporary tracking control takes place to evaluate the deviation of the head position from the target track by reading the current track address. In order to correct the deviation from the target track, the fine positioning mechanism such as a galvanomirror mounted on the optical head is activated to move the light spot on a track-by-track basis, and the light spot is finally positioned to the target track.
The optical disk storage takes an access time which is the sum of a seek time in which the recording/reproduction light spot is moved to a target track and a time in which the disk rotates until a target sector is reached. The disk rotation time is determined in many cases from the data transfer rate of the system, and therefore the key factor for the reduction of the access time is to minimize the seek time. Particularly, a high-speed seek operation takes a longer settling time relative to the coarse seek time, and it cannot be neglected. In case a high-speed disk has a larger eccentricity, it will take a considerable time before the normal tracking takes place after the tracking servo system has been activated.
Besides the two-stage seek control involving the coarse seek and fine seek as described above, there is another seek control known as cross tracking. The cross tracking control, as disclosed in the above-mentioned Japanese Patent Unexamined Publication No. 58-91536, is to position the optical head to a target track using a coarse actuator by counting track traverse pulses each generated when the head moves across a track. However, in a high-speed seek operation, the header and data signals written on the disk intermingle with the tracking error signal, and the coincidence of its band with the band of track traverse pulses causes erroneous counting of tracks, and therefore this seek control system is not suitable for the high-speed access operation.